CBN cutting tools having their cutting edges formed of cBN-based sintered bodies can perform cutting with high efficiency for a prolonged period or time, and are far more adaptive and environmentally friendly than grinding tools, because the cBN-based sintered bodies forming their cutting edges are chemically stable and high in hardness. Thus, CBN cutting tools are known as tools which make it possible to replace grinding of ferric workpieces that are difficult to grind, particularly workpieces made of hardened steel, with cutting. Today, as a result of increased rigidity of machine tools and improvement in the material of cBN-based sintered bodies, CBN cutting tools are being used for rough machining of e.g. vehicle transmission parts of steel at a feed rate f exceeding 0.3 mm/rev after hardening, and even for finishing of such parts, in which a surface roughness of 3.2 micrometers in ten point height of irregularities (Rz) is required.
Today, due to increasing tendencies to produce goods in greater varieties, each variety in a smaller lot, and increasing environmental concerns, CBN cutting tools which can be used for further high-efficiency machining, in which the cutting resistance increases, and for dry machining, in which the temperature of the cutting edges tend to increase, are desired. It is further desired that such CBN cutting tools exhibit satisfactory cutting performance with a minimum area of the expensive cBN-based sintered bodies.
In conventional CBN cutting tools, of which each cBN-based sintered body has an arcuate nose, flanks, a rake face and a chamfer having a uniform width and extending parallel to the cutting edge, it is possible to increase their durability against the cutting load in the rotational direction of the workpiece (i.e. direction of the principal cutting force) to some extent by increasing the chamfer width CW and chamfer angle γ (see FIG. 9) of the chamfer 13 (typically to 0.1 to 0.2 mm and 15 to 45 degrees, respectively), as disclosed in Patent publications 1 and 2. But there are few effective ways to increase the durability against the cutting load in the feed direction of the tool or in the direction of the thrust force, which is produced during high-efficiency machining at a high feed rate or with a large depth of cut. In fact, increasing the radius of curvature of the nose is practically the only way to increase such durability.
Cutting tools having their cBN-based sintered bodies brazed to the substrate are widely used because it is possible to minimize the area of the cBN-based sintered bodies and thus the tool cost is low. In such cutting tools, in order to ensure sufficient brazing strength, the radius of curvature of the nose has to limited to up to about 2.0 mm. Thus, even if the radius of curvature of the nose is increased to the upper limit of 2.0 mm, the durability against large stresses applied to the cutting edge during high-efficiency machining at a high feed rate is not sufficient. Moreover, when subjecting hardened steel workpieces having a complicated shape such as vehicle transmission parts to copying and recess forming, it is often necessary to use inserts of which the noses have a radius of curvature of 0.8 mm or less.
Further, it is urgently required today to develop cutting tools which can form a high-quality machined surface on a workpiece during a finishing step which determines the fatigue strength and sealability of the machined surface, instead of by grinding, which is limited in terms of machining efficiency and flexibility.
In conventional such cutting tools, the geometric shape of the cutting edge is transferred onto the machined surface, so that it was inherently difficult to obtain a desired surface roughness. Also, in such conventional cutting tools, it was impossible to positively utilize compressive stresses, which improve the fatigue life of the workpiece.
In order to solve these problems, Patent publication 3 proposes a CBN cutting tool for high-efficiency, high-precision machining which includes cutting tips each having an arcuate nose and an end cutting edge having a wiper edge (12 in FIG. 10). Such tools are hereinafter referred to as wiper tools.
Theoretically, such a wiper tool can provide good surface roughness while the feed rate is below the width WW of the wiper edge. But in high-efficiency machining at a feed rate or with a large depth of cut, the durability against the cutting load in the feed direction of the tool or in the direction of the thrust force is no higher than the durability achieved by conventional tools having arcuate noses. Thus, the cutting edges tend to chip.
For the properties of the machined surface too, in the case of a wiper tool, due to the burnishing action in which feed marks M (see FIG. 3(a)) formed by the transfer of the shape of the cutting edge are repeatedly pressed and flattened, tensile stresses produced by thermal stresses relax, so that residual stresses in the rotational direction of the workpiece tends to be suppressed compared to conventional tools with arcuate noses.
For residual stresses in the tool feed direction, however, their levels are often not substantially different between cutting using wiper tools and cutting using tools with arcuate noses. Thus, it was impossible to use wiper tools to machine sliding parts which are used in hostile environments, and parts for which complete sealability is required.
Patent document 1: JP utility model publication 64-34103A
Patent document 2: JP patent publication 8-318411A
Patent document 3: JP patent publication 2003-175408A